Wireless communication systems typically include a plurality of base stations where one or more base stations serve a geographic area commonly referred to as a cell. Users of the wireless communication system use their user equipment (UE) to communicate with each other and/or with the communication system. The communication system operates in accordance with established rules promulgated by governmental and industry groups; these rules are established as standards with which wireless communication systems comply. The standards contain various protocols that dictate the operation of the wireless communication systems. A protocol is a set of rules in accordance with communication within a communication system is to be initiated, maintained and terminated. The equipment at the base stations are referred to as system equipment. In wireless communication systems, communication can occur between two UEs (e.g., two cell phones or mobiles, wireless computer) or between a UE and system equipment.
A UE located in a particular cell being served by a base station communicates with other UEs or with other communication systems via the base station. The signals from the UE are transmitted to a receiving UE or to another communication system via the base station of the cell within which the UE is located. As a UE moves through cells being served by different base stations, communication from the UE is handled by the different base stations. Communication for a UE moving from one cell to another cell is transferred from one base station to another base station of the cell. The process in which communication for a UE is transferred from one base station (i.e., original base station) to another base station (i.e., target base station) is known as “handoff.” A UE located at the edge of a serving cell and near a target cell will request a handoff to the target cell depending on the relative strength of communication signals of neighboring base stations. In many wireless communication systems, the base stations transmit pilot signals periodically and such pilot signals are received by the UEs which measure the strength of these pilot signals. A UE will request a handoff to one or more base stations based on the relative measured strengths of the pilot signals of the base stations.
One particular type of handoff used in many wireless communication systems such as CDMA (Code Division Multiple Access) systems is a soft handoff. In a soft handoff, communication with the target base station is established before communication with the original base station is terminated. Also during the soft handoff, the UE is in simultaneous communication with more than one base station. In this manner, no interruption of communication to and from the UE occurs. Eventually, when the process of soft handoff has ended, the UE communicates with another UE or another communication system via the target base station.
A soft handoff in CDMA systems occurs based on the pilot signal strength of several sets of base stations as measured by the UE. These sets are known respectively as the active set, the neighbor set, the candidate set and the remaining set. The active set is the set of base stations through which active communication is established. The neighbor set is a set of base stations surrounding the active base stations and includes base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication, but through which active communication is not yet established. The remaining set is a set of base stations that are not included in any of the other three sets. Typically, the active set members have the higher measured pilot signals strengths relative to the other sets. The UE uses these sets to control the handoff process. When communications are initially established, a UE communicates through a first base station which is included in the active set.
During handoff however, the active set contains more than one base station. The UE monitors the pilot signal strength of the base stations in the active set, the candidate set, the neighbor set and the remaining set. During handoff when a pilot signal strength of a base station in the neighbor or remaining set reaches a defined threshold level, that base station is added to the candidate set and removed from the neighbor or remaining set by the UE. When the UE detects a relatively strong candidate pilot, the UE transmits a “Pilot Strength Measurement Message” (PSMM) (also known as a ‘UE Measurement Report’) to a Radio Network Controller (RNC) along with a request to add the base station of that pilot signal to the UE's active set. The UE Measurement Report is evaluated by the RNC which coordinates the processing of a soft handoff with the base stations associated with the strong detected pilot signals.
The RNC comprises radio and processing equipment that are able to communicate with the base stations and the UE. The RNC equipment are typically located at a Message Switching Center (MSC) of the wireless communication system. The UE Measurement report contains the measured pilot strength for a base station that the UE is requesting to be included in its active set. The RNC will determine if the base station, which the UE wants to be included in the UE's active set, has the requisite resources to serve the UE. If the base station requested by the UE has been determined by the RNC to have the requisite resources (e.g., transmit power, bandwidth, data rate) the RNC sends a message to the UE informing the UE that the base station requested by the UE can be added to the UE's active set. The UE sends a confirmation message back to the RNC confirming that the UE's active set has been updated and communication with the base station just added to the active set can commence. Thus, during handoff, the UE is in simultaneous communication—via a communication link comprising one or more communication channels—with the active set members. The communication link between a UE and a base station is often referred to as a leg. Eventually, the handoff process transfers communication to one of the base stations in the active set. The UE Measurement Report and other messages conveyed between a UE and a RNC and between a base station and a RNC during a handoff procedure or immediately prior to handoff are messages associated with a handoff or soft handoff.
In practice it is common for UE Measurement Reports messages to occur in bursts, particularly at the edges of areas of cell coverage where there are multiple strong candidates detected by the UE. The RNC, which processes the UE Measurement Reports during soft handoffs, typically handles the UE Measurement Reports in the order received, without preference, and does not process a subsequent UE Measurement Report message while processing a previous message. For example, when a UE detects two strong candidate pilots B and C at slightly different times, the UE transmits two separate UE Measurement Reports to the RNC. While the first UE Measurement Report is being processed, the second UE Measurement Report cannot be processed. Furthermore, in the instance where the second UE Measurement Report is related to a stronger more preferable signal, the processing delay caused by the controller having to process the first UE Measurement Report before the second UE Measurement Report, can result in dropped calls. Further the end to end latency in the process of adding or dropping multiple legs can result in excessive transmit power from the base stations, reduced system capacity and relatively high error rate.
FIG. 1 is a time diagram that depicts conventional processing of messages by a RNC during soft handoff for a CDMA (Code Division Multiple Access) wireless communication system. In particular, FIG. 1 depicts the timing of the flow of messages between an RNC, two base stations (B, C) and a UE. As is depicted therein, a first detected pilot signal for a first base station B results in a UE Measurement Report 102, transmitted to the RNC by the UE. A second detected pilot signal for a second base station C results in a second UE Measurement Report 104 transmitted to the RNC by the UE. In this example, the UE is requesting that base stations B and C be added to its active set meaning that a leg between it and base station B and another leg between it and base station C be added. Once received, each of UE Measurement Reports 102 and 104 is processed separately by the RNC.
The RNC's processing of the first UE Measurement Report 102 is as follows. In response to the first UE Measurement Report 102, the RNC sends a setup request message 106 to base station B. The setup request message is a request from the RNC to base station B asking base station B whether it has the requisite resources to establish a leg between it and the UE. Base station B responds by sending a setup response message 108 to the RNC. The setup response message from B may be a confirmation from B that it is able to establish a leg between it and the UE and has done so. The setup response message from B may also be that it does not have the requisite resources to set up a leg; if so, base station B will not become part of the UE's active set. However, when base station B is able to confirm its ability to establish a leg between it and the requesting UE and informs the RNC via the setup response 108, an active set update message 110 is transmitted to the UE by the RNC. The Active Set Update message 110 is an authorization from the RNC authorizing the requesting UE to add base station B to its active set. The UE adds base station B to its active set and then sends an active set update complete message 109 to the RNC informing the RNC that base station B is now part of the UE's active set. The RNC keeps its own active set record of the UE and therefore updates the active set record for that UE to include base station B. The UE is now able to communicate with base station B.
After the RNC receives Active Set Update Complete message 112 from the UE, the RNC begins to process UE Measurement Report 104, by transmitting a Setup Request message 114 to base station C. Similar to the responses to Measurement Report 102, base station C sends a setup response message 116 to the RNC. The RNC then sends an Active Set Update message 118 to the UE, which in turn updates its active set and transmits an Active Set Update message 120 to the RNC, thereby completing the processing of UE Measurement Report message 104, viz., adding base station C to the UE's active set. The time delay between the time that the second UE Measurement Report 104 is received by the RNC and the time that actual processing of this message begins (after message 112 is received by the RNC) can be so long that the UE misses the pilot signal associated with the second UE Measurement Report which could result in a dropped call.
Thus, in conventional systems, many times dropped calls occur (even when a strong pilot signal is present and has been detected) because of the delay caused by serial signal processing of UE Measurement Reports by the RNC. There is a need, therefore, for more efficient processing of messages associated with soft handoffs in wireless communication systems.